This invention relates generally to epitaxial growth techniques, and more particularly, to growth of Group II-VI semiconductor crystalline materials.
As is known in the art, Group II-VI semiconductor epitaxial materials such as cadmium telluride and mercury cadmium telluride have important applications as photodetector elements for detection of electromagnetic energy in the spectral range from approximately 0.8 .mu.m to 30 .mu.m. By adjusting an alloy composition of Cd and Hg, photodetector elements comprised of mercury cadmium telluride may be fabricated to cover selected wavelength ranges within the 0.8 .mu.m to 30 .mu.m wavelength band. For short wavelength applications, i.e., those wavelengths close to 0.8 .mu.m, cadmium telluride (CdTe) is generally used. For longer wavelengths greater than approximately 0.8 .mu.m, mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) is used where x is the alloy composition of cadmium to tellurium. By adjusting the alloy composition, an epitaxial material is provided which is responsive to selected portions of the entire spectral range of 0.8 .mu.m to over 30 .mu.m. That is, photodetector elements may be fabricated to be responsive to particular wavelength bands of infrared electromagnetic energy.
Several different techniques have been suggested for providing epitaxial layers of cadmium telluride and mercury cadmium telluride suitable for use in photodetector applications.
One method suggested to grow the epitaxial layers of HgCdTe and CdTe is metalorganic vapor phase epitaxy (MOVPE), also referred to as metalorganic chemical vapor deposition (MOCVD). The MOCVD technique involves directing vapors of primary alkyls of Group II and Group VI elements into a reactor vessel and chemically reacting the metalorganics to provide the epitaxial Group II-VI material. As described in an article entitled "The Growth of Cd.sub.x Hg.sub.1-x Te Using Organometallics" by J. B. Mullin et al, Journal of Vacuum Science Technology, Volume 21, No. 1, May-June 1982 [1], and as described in an article entitled "Vapor Phase Epitaxy of Cd.sub.x Hg.sub.1-x Te Using Organometallics" by J. B. Mullin et al, Journal of Applied Physics, Volume 14, 1981, pps. L149-151 [2], while heretofore epitaxial growth of mercury cadmium telluride epitaxial films has been demonstrated, as indicated in these articles, the quality of the epitaxial films grown is not suitable for use in the photodetector applications. Such materials are not suitable because they have relatively high carrier concentrations and the electron mobilities of the material are relatively low. These two properties are important in photoconductive detector elements, because such elements act as infrared sensitive resistors having a predetermined conductivity between a pair of contacts thereof, which changes in response to incident infrared energy. If the carrier concentration is too high, or the electron mobility is too low, carriers generated in response to the incident infrared radiation will recombine without the conductivity of the device changing between the terminals.
MOCVD growth of Group II-VI materials, for example, HgCdTe, includes the steps of directing primary alkyls of cadmium and tellurium, for example, dimethylcadmium, diethyltelluride, and a source of mercury into a reactor vessel where these materials react and form the HgCdTe. The Hg source is provided either by a metalorganic Hg compound such as dimethylmercury, diethylmercury or by elemental Hg. Elemental Hg is the preferred source since it is generally more pure than any of the metalorganic sources and is generally less expensive. Further, elemental Hg is less toxic than known metalorganic sources, since elemental mercury has a vapor partial pressure some 10.sup.4 times lower than the vapor pressure of known metalorganic Hg sources.
One problem encountered during growth of HgCdTe epitaxial layers is the relatively high growth temperatures required and relatively slow growth rates which heretofore have been achieved. In a third article entitled "The Growth by MOVPE And Characterization of Cd.sub.x Hg.sub.1-x Te" by J. C. Irvine et al, Journal of Crystal Growth, Volume 55 (1981), pps. 107-115 [3], it was described that during growth of HgCdTe epitaxial layers, control of the Hg partial pressure over the growth region at an elevated temperature in the range of 400.degree. to 500.degree. is very difficult. Control of the equilibrium Hg partial pressure is very important because it is the concentration of Hg vacancies in the epitaxial layers which provide carriers and, hence, control the electrical properties of the material. It was further reported in reference [3] that the Hg partial pressure has to be at least two orders of magnitude greater than the Te partial pressure in order to allow the reaction producing the HgCdTe to efficiently proceed. At 400.degree. C., the minimum Hg partial pressure for growth of HgCdTe is 0.01 atmospheres. Typically, this partial pressure of Hg is provided by heating the elemental liquid source of Hg and directing a carrier gas across the Hg to provide a Hg vapor stream.
One technique for providing Hg control is described in U.S. Pat. No. 4,568,379, filed Sept. 12, 1984, by William E. Hoke, Richard Traczewski and Peter J. Lemonias, and assigned to the same assignee as the present invention. There the Hg partial pressure, and hence the electrical properties of the HgCdTe epitaxial layers, were controlled by maintaining the temperature of the Hg in the incoming vapor stream through heating of the walls of the reactor to a temperature of at least 240.degree. C. While such a technique allows for control of the Hg partial pressure over the growth region by preventing premature condensation of Hg from the vapor stream onto the reactor walls, it also has been observed that, at this temperature, pre-reaction of the reactants may occur. While it has not been determined whether pre-reaction has any effect on the quality of the epitaxial layers, it appears nevertheless that the pre-reaction reduces the concentration of the reactants in the vapor stream, thus further reducing the growth rate.
It was also described in reference [3] that the low temperature limit in HgCdTe growth by MOCVD is the relatively high stability or low cracking efficiency of the organic reactants dimethylcadmium (DMCd) and diethyltelluride (DETe) which provide the sources of Cd and Te. It was also found that DETe is the more stable compound of the two, and the yield of Te was very much dependent on the temperature of which the DETe would dissociate. At temperatures or about 400.degree. C., the amount of Te which was freed from the DETe was relatively small. Accordingly, the growth rate of HgCdTe at 400.degree. C. is very low.
Several additional problems have also been shown with MOCVD growth of HgCdTe. A high concentration of reactants is required to provide a suitable growth rate of HgCdTe. Since the cracking efficiency of the reactants is low, there is a large waste of reactants. However, the reactants, particularly metalorganic sources of Cd and Te, are very expensive.
Therefore, the large waste of reactants makes epitaxial growth of such layers relatively expensive. Furthermore, the waste products are highly toxic, and, therefore, the large volume of waste reactants produced by typical prior processes poses a significant problem for safe disposal of the toxic waste products.
Moreover, at growth temperatures of 400.degree. C. there is significant interdiffusion and alloying of adjacent layers of different epitaxial materials. This problem complicates growth of well-defined heterostructures and abrupt electrical P-N junctions.